President Yusuf, 72, had a liver transplant in the 1990s and has not been in good health but his spokesman said his condition was not related to his liver.

Our problem is not with the old prime minister or the new one, our problem is Ethiopia's occupation

Sheikh Sharif AhmedIslamist leader

"The president is well and not in any danger as reported by the media, he is responding well to treatment and will be out of hospital soon. It is normal for anybody to be sick," Mr Mohamoud told the BBC News website.

Some health officials had described his condition as "serious".

He is due to travel to London this week, for what his aides are calling a "routine check-up".

Officials at the Nairobi Hospital where President Yusuf has been admitted have barred reporters from visiting the private ward.

But the Somali envoy in Kenya, Mohammed Nur Ali, said they plan to give a briefing to the media on his health condition.

President Yusuf will miss Wednesday's meeting of regional leaders with US Secretary of State Condoleezza Rice in Ethiopia.

'Serious'

Somalia is instead represented by Prime Minister Nur Hassan Hussein who was appointed just last month.

He has called for talks with the opposition leaders, hoping to end the year-long insurgency against the Ethiopian-backed transitional government, that has left at least 6,000 people dead.

Some one million Somalis are living rough, the UN says

But Sheikh Sharif Ahmed, chairman of the opposition Alliance For the Re-Liberation of Somalia (ARS), insists talks can not take place unless Ethiopia withdraws its troops from Somalia.

"Our problem is not with the old prime minister or the new one, our problem is Ethiopia's occupation," he told Reuters news agency.

Ethiopia helped the transitional government end the Union of Islamic Courts' (UIC) six-month rule over large parts of southern Somalia, last December.

Mr Ahmed was one of the UIC leaders.

On Monday, UN humanitarian affairs chief John Holmes said the international response to the situation in Somalia has been inadequate.

"This is obviously a very serious humanitarian situation in Somalia," he said.